Objectives: COVID-19 has become a major source of stress for people around the world. Stressful life events play a role in the pathogenesis of sleep disorders such as insomnia which is considered a risk factor for anxiety and depression. Emotion regulation is an important factor linked with sleep and mental health problems. Therefore, the main goal of the present study was to examine whether insomnia could constitute a mediation mechanism that explains the relationship between emotion regulation strategies (rumination, reappraisal, suppression) and stress-induced mental health outcomes during the COVID-19 pandemic. Material and Methods: The cross-sectional study was conducted among young (M±SD 24.8±2.24) individuals (N = 281, 85.4% women) during the time of the third wave of infections in Poland. Data were collected by means of selfreport questionnaires, including the Emotion Regulation Questionnaire; Cognitive Emotion Regulation Questionnaire; Athens Insomnia Scale; Depression, Anxiety and Stress Scale. The direct and indirect effects of emotion regulation strategies on depression, anxiety, and stress were calculated using a bootstrap estimation technique. Results: All analyzed indirect effects were significant. The results show that insomnia mediates the relationships between all 3 emotion regulation strategies and stress, anxiety and depression. Conclusions: The presented results shed the light on the role of insomnia on the relationships between emotion regulation strategies and emotional states experienced during the COVID-19 pandemic. Additionally, the present study suggests that educational and therapeutic interventions aimed at improving emotion regulation might be useful for improving symptoms of insomnia and, through it, symptoms of affective disorders. Int J Occup Med Environ Health. 2023;36(1):151–59